Rubber compositions, particularly diene elastomer based rubber compositions, are sometimes prepared with particulate reinforcement comprised of precipitated silica which may also include rubber reinforcing carbon black. Such rubber compositions often also contain silica coupling compounds to couple the precipitated silica to diene based elastomers contained in the rubber composition which are sometimes referred to as silica couplers or silica coupling agents. Such silica couplers usually contain a moiety reactive with hydroxyl groups (e.g. silanol groups) contained on the precipitated silica and another different moiety interactive with the diene based elastomers in the rubber composition.
Historically, such silica couplers have been provided as being comprised of, for example, alkoxyorganomercaptosilanes and bis(3-triethoxysilylpropyl) polysulfides having an average of from about 2 to about 4 connecting sulfur atoms in their sulfidic bridge. For the alkoxyorganomercaptosilanes, because, in part, they normally present a particularly high chemical activity with diene based elastomers which, in turn, can lead to rapid viscosity build-up of a rubber composition while being mixed in an internal rubber mixer, various alkoxyorganomercaptosilane coupling agents are often introduced into the rubber composition with their mercapto group being reversibility blocked to thereby render the mercaptosilane latent in a sense of having a significantly reduced, or possibly non-existent, chemical activity until they become unblocked by a chemical deblocking agent, which is usually accomplished while the blocked mercaptosilane is being mixed with the rubber composition.
For this invention, it is proposed to evaluate use of a siloxy trithiocarbonate as a coupling agent, namely as a blocked coupling agent, which can become chemically unblocked by reaction with a primary amine by mixing a combination of the blocked coupling agent with a primary amine compound in the rubber composition.
By such method, an unblocked alkoxymercaptosilane based silica coupling agent is formed within the rubber composition, and it is desired to evaluate coupling the precipitated silica by such unblocked coupling agent to diene based elastomer(s) contained in the rubber composition to promote formation of a precipitated silica reinforced rubber composition.
Historically, siloxy trithiocarbonate has been reported as a free radical polymerization control agent, (U.S. Pat. No. 6,998,452), as well as anionic polymerization chain end terminator, (U.S. Pat. No. 8,865,829). U.S. Pat. Nos. 6,579,949 and 8,217,103 discuss use of molecules based on protected sulfur as anionic polymerization terminating agents. U.S. Pat. Nos. 6,229,036 and 6,608,125 report molecules containing protected sulfur as coupling agents between polymer and filler.
In the description of this invention, the term “phr” where used means “parts of material by weight per 100 parts by weight of rubber”. The terms “rubber” and “elastomer” may be used interchangeably unless otherwise indicated. The terms “rubber composition” and “compound” may be used interchangeably unless otherwise indicated.